Hi,

I'm using the SN74LS123 one-shot to lengthen a pulse. The chip works fine when my input pulse widths are > 500ns. I would like to input a 100ns pulse & have the chip trigger off of it.
I'm not sure why it doesn't ? I've been trying to find the spec on minimum input pulse width but no luck so far. Does anyone know ?

Regards,

 

My Motorola Schottky TTL manual does not show a minimum trigger pulse width. Are you sure the pulse meets the TTL logic level requirement?

 

Read through this thread http://forum.allaboutcircuits.com/showthread.php?t=5678

Post your schematic of the circuit.

 

I'm pretty sure the input pulse is TTL-compatible, because I'm using a function generator to supply the input pulse. I setup everything & then start to decrease the pulse width. For example, at 1us it works fine. Finally after stepping down to ~ 400ns, the output is zero (no pulse).

Attached is the schematic.

 

I would suggest you reconfigure your 50K resistor to be a variable resistor and connect it between the R/C pin and Vcc. It will not solve your minimum input pulse width problem but it will make the output pulse width adjustment better.

hgmjr

 

Questions:

1. Which test results that lead you to believe it's not working as you designed? I mean, within a few nanoseconds after the input pulse, did Q go high or did it remain low? Can you post the pic of your oscilloscope presentation?

2. What model oscilloscope are you using to assist you with your troubleshooting?

3. Your delayed output seems abnormally long. Do you need that type of a delay? What is your variable resistor set at?

 

In pdf spec at the link below, under general operating conditions, it specifies a pulse duration of 40 nanseconds. I'm not sure if this refers to the input pulse.

74LS123 Datasheet pdf - RETRIGGERABLE MONOSTABLE MULTIVIBRATORS

- Texas Instruments

John

 

chesart1 - the 40ns number refers to tw, the output pulse

hgmjr - he 50K is a pot & is hooked up between R/C & Vcc

joe - yes, the RC time constant is large - I need an output pulse (Q) of many seconds - & I'm getting it. The problem only happens when I start decreasing the input pulse width. The output pulse simply disappears (the chip no longer reacts to the trigger pulse).

 

You could try with the 74HC/74HCT logic family. It is even faster than the 74LS family.